I work as a signing agent and utilize the 'Sign Here' stickers for the legal documents I prepare for signing. These are very visible and easy to re-use, but can stick to the point of tearing the document sometimes when they are removed.

I prepare and process a lot of tax returns for different clients. I have used these postits for the last ten years. Previously, I had to write similar instructions on piece of paper and attach. These attach to the document, do not deface in any way, and tell the client exactly where to sign.